1. Field of The Invention
The present invention relates to an X-ray mask for manufacturing semiconductor devices such as integrated circuits (IC) and very large scale integrated (VLSI) circuits. More particularly it is concerned with an X-ray mask, suitable for an exposure apparatus for manufacturing semiconductor devices that uses a soft X-ray having wavelengths of about 2 .ANG. to about 150 .ANG..
2. Related Background Art
In recent years, in exposure apparatus for manufacturing of semiconductor devices such as IC and VLSI, many kinds of exposure apparatus utilizing soft X-rays and capable of obtaining prints with a higher resolution have been proposed as semiconductor devices have become more highly integrated.
In general, the X-ray mask used in this soft-X-ray apparatus comprises a support frame having the shape of a ring, and a film member stretched to cover its opening and having X-ray transmitting portions and X-ray non-transmitting portions.
The non-transmitting portions are formed of an X-ray opaque material (including an absorber) having a geometrical pattern and provided on a support membrane (a mask membrane) that constitutes a substrate in said film member. The X-ray opaque material with a submicron size is provided on a wafer surface. On the other hand, the transmitting portions are formed of the mask support membrane itself corresponding to the part on which the light shielding material is not provided.
In the exposure apparatus using soft X-rays, both the space in which the X-ray mask is disposed and the upper space on the resist coated side on a wafer, in many instances, are put into a reduced pressure atmosphere or a low-pressure helium atmosphere in order to prevent the absorption loss of energy of the light irradiated from a soft-X-ray source for exposure.
If exposure is carried out with use of soft X-rays having wavelengths, for example, of approximately from 2 to 150 .ANG. under such conditions, the X-ray mask support membrane, a mask absorber and atmospheric gas atoms absorb the soft X-ray and emit photoelectrons by the photoelectric effect. In particular, when the support membrane comprises an insulator as the thin membrane, such exposure causes not only electrostatic charging owing to the emission of photoelectrons from the constituent atoms of the support membrane but also electrostatic charging owing to the emission of photoelectrons from the absorber having a large number of electrons, resulting in a support membrane having a high positive potential.
In general, the distance between the support membrane and the resist surface on the wafer is so small that the run-out error caused by the divergence light from the source can be negligible, and is set, for example, in the range of from 10 .mu.m to 100 .mu.m. For this reason, the electrically charged support membrane and wafer may electrostatically attract each other, resulting in deformation of the support membrane, so that it sometimes occurs that the exposed pattern precision is lowered because of a flexure of the support membrane or that the support membrane comes into contact with the wafer when the attraction force is particularly strong.
In exposure apparatus aiming at obtaining a high resolution such that the pattern size to be transferred is 0.5 .mu.m or less, the support membrane is commonly so constituted that it may have a thickness of about 2 .mu.m, using an inorganic material (ceramics in particular) having a small thermal expansion coefficient and large Young's modulus. In this way misregistration due to its thermal expansion or the distortion of absorbers due to residual stress can be suppressed.
In general, many of materials for this purpose are brittle and hard. Hence, it sometimes occurs that the support membrane is easily broken by excessive deformation or application of nonuniform pressure.
As a means to settle this problem, there is, for example, a method in which a metal having small absorption to X-rays, as exemplified by aluminum, is deposited on the surface to a thickness of about several hundred .ANG.. This method can achieve a good electric conductance and good soft-X-ray transmittance, but has the problem that the transmission of visible or infrared light for achieving alignment between the mask and the wafer is so extremely low that precise alignment may be extremely difficult.